1. Field of the Invention
The present invention relates to a polymerizable composition which is curable by exposure to heat or infrared rays and a planographic printing plate precursor using the polymerizable composition. Particularly, the present invention relates to a polymerizable composition (including a novel compound) being superior in curability and a negative type planographic printing plate precursor provided with a recording layer comprising the polymerizable composition and having high curability.
2. Description of the Related Art
The development of lasers in recent years has been remarkable. Particularly, as to solid lasers and semiconductor lasers (hereinafter may be referred to as “infrared laser” which radiate infrared whose wavelength is in a range of 760 to 1200 nm, high output and small-sized lasers are now easily available. In a planographic printing field, in particular, these infrared lasers are very useful as a recording light source when a printing plate is directly made by digital data of, for example, computers. Recently, there has been an increased demand for image recording materials consisting of a highly sensitive polymerizable composition, specifically, for negative type image recording materials whose solubility to a developer is greatly deteriorated by exposure to infrared rays.
Generally, negative type image recording materials which are recordable by exposure to infrared rays contain; a photothermal converting agent which absorbs infrared energy and converts the energy into heat; a polymerization initiator, generating radicals by the heat generated by this photothermal converting agent and; a polymerizable compound which is polymerized by the action of the generated radicals as a polymerization initiator. Such negative type image recording materials utilizes a recording system in which the recording layer (image recording materials) of the exposure portion is cured along with the initiation and progress of a polymerization reaction of this polymerizable compound, to form an image. A negative type image recording material like this has poorer image forming ability than a positive type image recording material in which the recording layer is solubilized by the energy of infrared light. Therefore, in order to promote a curing reaction by polymerization to form a strong image portion, such a negative type image recording material as described above is usually subjected to heat treatment prior to a developing step.
An image recording material comprising a photothermal converting agent, an acid generator, a resol resin and a novolac resin is described in U.S. Pat. No. 5,340,699. Such a negative type image recording material must also be subjected to heat treatment after exposing to an infrared laser for the formation of an image. Therefore, a negative type image recording material which needs no heat treatment after being exposed has been desired.
As a negative type recording material which needs no heat treatment, a thermal polymerization based image recording material using, as the polymerization initiator, a known polymerization initiator such as peroxides and azobisnitrile type compounds is known (see, for example Patent Document 1). When using these known azo compounds or peroxides or the like, although image forming ability is improved, there arises a problem that recording sensitivity and printing durability after developing are unsatisfactory, and in the case of using a highly sensitive polymerization initiator, there arises another problem that an undesirable polymerization curing reaction proceeds due to the influence of the storage environment, whereby contamination is likely to occur in the non-image portion of a print product.
Also, a recording material, which comprises a cyanine dye having a specific structure, an iodonium salt and an addition-polymerizable compound having an ethylenic unsaturated double bond and needs no heat treatment after image-wise exposure, is known (see, for example, Patent Document 2). This image recording material, however, has the problem that polymerization inhibition is caused by oxygen in air at the time of a polymerization reaction, leading to a reduction in sensitivity and insufficient strength of a formed image portion. A composition containing an ethylenic unsaturated compound and a polymerization initiator provided with an onium salt having a specific counter anion has been proposed, to solve these problems (see, for example, Patent Documents 3, 4 and 5). However, still higher printing durability and sensitivity are needed at present.
[Patent Document 1]
                Japanese Patent Application Laid-Open (JP-A) No. 8-108621[Patent Document 2]        Japanese Patent Publication (JP-B) No. 7-103171[Patent Document 3]        JP-A No. 2001-34742[Patent Document 4]        JP-A No. 2002-6482[Patent Document 5]        JP-A No. 2002-148790        
On the other hand, among those which are selectively sensitive to infrared light, as the positive type image recording material whose solubility at an exposed portion (non-image portion) to an aqueous alkali solution is increased, a heat-responsive resist material making use of a phase change of a novolac resin is known (see, for example, Patent Document 6). However, the heat-responsive resist material is easily scratched and thus has a problem in its handling.
As a negative type image recording material having high scratch resistance, an image recording material which is provided with a recording layer containing an infrared absorber, an onium salt and a polymerizable compound and needs no preheating when an image is formed, has been proposed (see, for example, Patent Document 7). According to this technique, a certain degree of printing durability is obtained, even if preheating is not conducted. However, a negative image recording material of this type has a problem in that printing durability is deteriorated particularly when a cleaner is used, and thus printing durability thereof must be enhanced.
[Patent Document 6]
                JP-A No. 9-43847[Patent Document 7]        JP-A No. 2001-133969        
A compound having a COCOO group, which compound solves the above-mentioned problems in image forming materials using the conventional polymerizable composition, is known as a photo (polymerization) initiator. However, this polymerization initiator has insufficient sensitivity and therefore studies of enhancing the sensitivity thereof has been made. As the compound having a COCOO group, a polymer containing a COCOO unit is known (see, for example, Patent Document 8). The COCOO unit described herein has a COCOOH group or a structure such as COCOO—NR4+ and therefore has only insufficient sensitivity as a photoinitiator.
Also, an image formation method using a radical generator such as azo compounds and peroxides is also known (see, for example, Patent Document 9). However, the radical generator used herein has insufficient sensitivity and also has a problem concerning a long-term stability.
Also, a recording material which comprises a cyanine dye having a specific structure, an iodonium salt and an addition-polymerizable compound having an ethylenic unsaturated double bond and needs no heat treatment after image-wise exposure is known (see, for example, Patent Document 10). This image recording material, however, has a problem that polymerization inhibition is caused by oxygen in air at the time of a polymerization reaction, leading to a reduction in sensitivity and insufficient strength of a formed image portion.
To solve these problems, compositions containing a polymerization initiator provided with an onium salt having a specific carboxylic acid anion and an ethylenic unsaturated compound is known (see, for example, Patent Documents 11 and 12). These compositions, however, have problems concerning curability (high printing durability) and sensitivity. Therefore, a polymerization initiator which can generate radicals by light or heat in a highly sensitive manner, can rapidly cause a curing reaction and make the reaction proceed has been desired.
[Patent Document 8]
                U.S. Pat. No. 40,465,778[Patent Document 9]        JP-A No. 8-108621[Patent Document 10]        JP-B No. 7-103171[Patent Document 11]        JP-A No. 2001-34742[Patent Document 12]        JP-A No. 2002-148790        